The present invention relates in general to active bolsters for occupant crash protection in automotive vehicles, and, more specifically, to increasing robustness of the bonding between an expandable bladder member and a trim wall which are separately molded and then hot welded together.
An active bolster is a vehicle occupant protection device with a gas-inflatable bladder to absorb impacts and reduce harm to occupants during a crash. As opposed to deployable air bag cushions that emerge from behind various openings upon inflation, active bolsters use the interior trim surface itself to expand at the beginning of a crash event for absorbing the impact and dissipating energy through deflation of the bladder. U.S. Pat. No. 8,205,909, issued Jun. 26, 2012, incorporated herein by reference, discloses an active knee bolster integrated into a glove box door that is light weight and visually attractive. U.S. Pat. No. 8,474,868, issued Jul. 2, 2013, also incorporated herein by reference, discloses a typical structure wherein an active bolster includes an outer wall or trim panel that faces a vehicle occupant attached to an inner wall or panel along a sealed periphery. One or both of the walls is deformable in order to provide an inflatable bladder. For example, the inner wall (i.e., bladder wall) may have a pleated (i.e., accordion-like) region that straightens out during inflation.
The inner and outer walls of a typical active bolster are comprised of molded thermoplastics such as polyethylene, polyolefin, or PVC. They are typically injection molded but could also be blow molded. When formed separately, the walls must be hermetically joined around their periphery in order to form the inflatable bladder. The joint must be strong to resist separation as a result of the high pressures during inflation and occupant impact.
A known method of sealing the bladder walls is by hot welding, which involves heating of the matching surfaces and then compressing them together. Examples include hot plate welding, IR welding, and laser welding. A generally planar welding flange has been provided around the outer perimeter of an inner (bladder) wall which is received by a generally planar surface of an outer (trim) wall. The outer wall and/or inner surfaces may also include upstanding welding ribs that increase the weld strength by penetrating and fusing with the welding flange or other opposing surface during the hot welding process in which the areas to be welded are heated and then compressed. Despite the penetration of these ribs, weld separation has continued to be a potential weakness for active bolsters for various reasons including warping of the parts to be welded, temperature variations of the hot plates during welding, uncorrected variances in the pressure applied during welding, and characteristic differences between the trim wall and bladder wall which may be molded from different plastic formulations.
Various stresses during inflation contribute to the possibility of weld separation. Due to the elasticity required for making the pleated bladder wall inflatable and the orientation of the hot weld seam, a significant peel stress may be applied to the weld seam during expansion. More specifically, the typical orientation of the weld seam involves a generally planar sheet which extends perpendicularly with respect to the outward expansion direction of the bolster. As the pleats on the bladder member unfold and the trim panel moves outward, the inner edge of the weld seam tends to be pulled apart by a peeling force. The weld seam may typically withstand greater forces in the shear direction (i.e., directed along the plane of the weld seam) than in the peel direction. However, the conventional orientation of the weld seam has advantages in cost and ease of manufacturing.